1. Field of the Invention
The present invention relates to a device for testing an analog-to-digital converter (ADC).
2. Discussion of the Related Art
A conventional analog-to-digital converter test comprises determining the signal-to-noise and distortion ratio (SINAD) of the converter. Term “distortion” corresponds to the frequency components present in the output signal other than the frequency components of the input signal. Such additional frequency components originate from the fact that the converter is not perfectly linear. This can be obtained via a test device temporarily connected to the converter and which provides the converter with a sinusoidal signal, stores samples provided by the converter, and determines, based on the memorized samples, the spectrum or frequency content of the digital signal provided by the converter from which the signal-to-noise and distortion ratio is determined. The spectrum or frequency content can be obtained by the determination of the fast Fourier transform of the digital signal provided by the converter.
A disadvantage of such a test is that the determination of the fast Fourier transform requires acquiring a significant number of samples, for example, from 10,000 to 20,000 in the case of an audio application. Further, the duration of a test implementing the fast Fourier transform determination is generally not negligible. As an example, for an audio application for which the sampling frequency of the converter is equal to 48 kHz, the duration of a test is generally greater than 1 second. Knowing that the cost of a test is directly linked to its duration, the cost of a test implementing the determination of a fast Fourier transform is generally high as compared with the cost of the actual circuit.
In many applications, an analog-to-digital converter of Σ-Δ type is used. Such a converter comprises a Σ-Δ modulator, which provides a generally binary signal at a frequency greater than the converter sampling frequency, and a filtering and decimation stage which provides a digital signal coded over several bits at the sampling frequency and eliminates the quantization noise outside of the useful signal band. An advantage of such a converter is that it can be entirely formed of a digital integrated circuit comprising no analog elements requiring accurate definition of their technical characteristics.
To decrease the cost of an analog-to-digital converter test, it would be desirable for the converter to comprise its own test device so that a test operation can be automatically implemented during the converter operation. In particular, when the converter is formed of an integrated circuit, it would be desirable for the test device to be made in integrated fashion to be associated with the converter. However, the forming of an integrated circuit performing the fast Fourier transform determination is relative complex and requires a significant silicon surface area, with a cost that may be exorbitant.